The 54 would be your best option imo because it will have the best BTS and give you the best cut. Maintenance should be simple enough, the oil filter and oil change easy enough on the two Kawasaki engines that I run and the FR isn't any different that I know of.
The floor (area where feet are placed) doesn't raise or flip up like there commercial models do so easy access to changing the belts isn't there like the commercials. Changing the belts isn't that difficult anyway.
As far as the lap bars go there adjustable to be raised or lowered to suit your comfort range. Two bolts in each handle can be removed to adjust their height. The only fore and aft adjustment would be moving the seat forward or back to adjust your mowing position. The SD comes with the 2800 drives which are the top of the line residential offerings so they should never give you any problems and do everything you'll ever want for your 2.5 acres.
The guard does come standard on the 60" and it is available for the 48 and 54" As far as the engine guard goes and this is just my opinion but I wouldn't waste the money because it's just something else to get in the way and it's just added weight and serves no purpose other than looks.
https://www.youtube.com/watch?feature=player_embedded&v=P_LvO0hA_TY
